Purpose <p>This study assessed hypothesized risk factors of sibling sexual abuse (SSA), drawing from two frameworks: Seto's Motivation-Facilitation model, which highlights the importance of individual and personality factors, and the evolutionary theory of incest aversion, which emphasizes the importance of generalized mechanisms to promote survival.</p> Methods <p>Data from 1,164 adults (18 to 30&#xa0;years old) were collected via an anonymous online survey. Using structural equation modeling, the fit indices of three hypothesized models of SSA were compared. The first model (the Motivation-Facilitation-Kinship model) combined elements of the Motivation-Facilitation model and evolutionary theory (i.e., kinship cues such as perceived resemblance between siblings). The second and third models excluded kinship cues or facilitation factors, respectively.</p> Results <p>The Motivation-Facilitation-Kinship model fit the data the best. Across all the models, motivation factors (e.g., atypical sexual interests) were most strongly related to the propensity for SSA. Contrary to evolutionary theory, we found that experiencing more kinship cues was related to an increased propensity for SSA. These findings were consistent regardless of the gender composition of the sibling dyad (i.e., same- versus other-gender).</p> Conclusions <p>This research did not find support for the evolutionary theory of incest avoidance, perhaps because kinship cues are better conceptualized as situational (access) factors, rather than facilitation factors. However, given the relationship between motivation factors and the propensity for SSA found in the present study, our results reinforce the importance of treatments focused on motivation factors, beyond other potential risk factors.</p>